Facebook should have given subscribers an opportunity to opt out of a controversial experiment in which the social network manipulated the emotional tone of people's news feeds, says the Salk Institute scientist who edits the journal in which the study's findings were published.

"We are not against the results (of the study). We think it is interesting science," said biologist Inder Verma, editor of the Proceedings of the National Academy of Sciences (PNAS). "But we think there should have been clearer procedures described (for Facebook users.)"

In January 2012, Facebook manipulated the news feed of 689,003 users, scrubbing some people's posts of good news and others of bad news. It was part of a study of contagious moods. The data was analyzed by Cornell University researchers, who found that people become somewhat more negative when they are exposed to less good news. Conversely, people were somewhat more positive when exposed to less bad news.

The study was published by PNAS in early June, setting off a firestorm of protest from Facebook users and privacy experts who say the social network was treating subscribers as unwitting guinea pigs in an experiment.

Verma noted that people give Facebook permission to experiment with things like the news feed when they sign up for the service. But he said that the user agreement is so long and complicated that most people don't examine it closely.

"I think people have the right to feel that they were manipulated," said Verma, adding that PNAS will examine future Facebook studies more closely to make sure that people were given the option to opt out of an experiment. Informed consent is a routine part of studies done by colleges, universities and most scientific institutions. It is referred to as the common rule.

"There are some people who say (PNAS) should retract the paper," Verma told U-T San Diego. "But we think that no one has been hurt in this case; no drugs were given, no actual procedures were done. So I think (a retraction) is going too far."

But he took the rare step of publishing an "editorial expression of concern" in the most recent issue of PNAS. The statement, written by Verma, says in part, "PNAS editors deemed it appropriate to publish the paper. It is nevertheless a matter of concern that the collection of the data by Facebook may have involved practices that are not fully consistent with the principles of obtaining informed consent and allowing participants to opt out."